The present invention pertains to preamplifiers for magnetoresistive elements. Specifically, it pertains to a preamplifier having a programmable bias reference current, electrical isolation among a plurality of multiplexed elements, and improved feedback control of a bias current source.
Magnetoresistive (MR) elements are especially useful in MR heads for reading high-density binary data stored on magnetic media. The resistance of MR elements depends on the magnitude and direction of an applied magnetic field. Hence, the resistance of an MR element changes as the magnetic field of an adjacent magnetic medium moves relative to the element. When the element is properly coupled to an amplifier, the amplifier senses, or detects, the resistance of the element as a voltage or current signal corresponding to the magnetic field of the medium. Other circuitry may then decode the signal to retrieve the stored data.
The resistance changes of MR elements are generally nonlinear. Thus, to improve utility, it is common to electrically and magnetically bias an MR element to operate within an optimal range of its operating characteristic. Electrical biasing entails applying a voltage or current to the element to establish a steady-state, baseline resistance against which resistance changes induced by the magnetic medium may be reliably detected. Electrical biasing requires coupling the MR element to a preamplifier.
In its basic form, the preamplifier comprises a bias circuit, which sets the baseline resistance of the MR element, and a differential amplifier, which is parallel-coupled to the element. Although the bias circuit may be as simple as coupling the element between two voltage or current sources, it may also include a feedback control system. The feedback control system measures, or senses, either a voltage or current of the MR element indicating an actual bias condition, compares the measured voltage or current to a desired voltage or current reference representing a desired bias condition, and adjusts the bias voltage or current source to achieve the desired biasing. Using feedback is advantageous because it maintains the desired biasing as the element varies with temperature, age, and wear.
One type of preamplifier for MR heads uses a feedback bias circuit having a current-bias-voltage-sense (CBVS) architecture. The CBVS architecture signifies a feedback bias circuit which controls a bias current source by sensing a voltage of the MR element. In feedback terms, the preamplifier compares an actual head voltage to a desired voltage and adjusts a current source according to a difference between the actual and desired voltages. The CBVS architecture provides two important advantages over other feedback architectures.
One advantage stems from using voltage sensing instead of current sensing to control the biasing. Current-sensing circuitry monitors the bias status of the head by using two conductive leads to parallel-couple the sensing circuitry to the head. This arrangement, however, is problematic, because the two leads typically have a low input resistance and parasitic inductance which combine to introduce a low-frequency pole into the transfer function of the amplifier parallel-coupled to the head. The low-frequency pole reduces the bandwidth of the amplifier. Voltage sensing, on the other hand, avoids this problem because voltage-sensing circuitry has a high input resistance. The high input resistance of the voltage-sensing circuitry effectively isolates this circuitry from the amplifier, leaving the bandwidth of the amplifier intact.
The second advantage of the CBVS architecture arises from using current biasing rather than voltage biasing. Current biasing entails biasing the head between a pair of balanced current sources. In other words, one terminal of the head is coupled to the output of a current source and the other to the input of a current sink. This biasing scheme permits maintaining the head at an arbitrary voltage while voltage biasing does not. Preferably, the arbitrary voltage equals the voltage of the disc surface to prevent arcing between the head and disc. Arcing can occur when the element comes close to the disc, if the voltage difference between the disc and the head is sufficiently large. Specifically, where the disc and head are separated by less than 10 micro-inches, a voltage difference exceeding a few hundred millivolts may cause arcing. Thus, it is desirable to maintain equality of the head and disc voltages to eliminate the risk of arcing. Current biasing facilitates this effort.
U.S. Pat. No. 4,870,610, issued to Jove et al. (Jove), discloses a preamplifier implementing the CBVS architecture. FIG. 1 is a block diagram of the Jove preamplifier, showing an MR head RH biased between dependent current source IBIAS and independent current sink IREF. A junction between equivalent resistors R1 and R2 derives a center voltage of head RH. A differential feedback amplifier A, comparing the center voltage to a reference voltage VREF, drives current source IBIAS. Differential output amplifier B amplifies a voltage across the terminals of head RH The preamplifier maintains approximate equality of the center voltage of head RH and reference voltage VREF by varying the current output from source IBIAS around a quiescent reference current set by current sink IREF. Accordingly, if head RH contacts or nearly contacts a magnetic disc having a potential equal to VREF, the low voltage difference between the element and the disc prevents arcing. Moreover, resistors R1 and R2 have large resistances compared to head RH, providing a high input resistance and effectively isolating the feedback loop from amplifier B. Thus, the Jove preamplifier provides the advantages of the CBVS architecture. It, however, is also beset by numerous problems.
One problem with the Jove preamplifier is that it does not permit programming the value of reference current sink IREF. As shown in FIG. 1, current sink IREF is an independent current sink. As such, its level of current input is not directly adjustable, thereby limiting the flexibility of the bias circuit to respond to changing head conditions. Moreover, the ability to independently define this current is particularly important in transducers comprising a plurality of MR heads multiplexed to common bias circuitry. Heads inevitably differ; therefore, to ensure consistent biasing and performance of all heads, the bias current should be tailored to each head. Accordingly, providing a convenient means for programming, or adjusting, current sink IREF would advance the art.
Another problem with the Jove preamp arises from using it with a plurality of selectable, or multiplexed, MR heads. FIG. 2 shows the Jove preamp configured for two selectable MR heads. For sake of clarity, the biasing circuit comprising current source IBIAS, current sink IREF, differential amplifier A, and related switching transistors for coupling selectively to heads RHA RHB are not shown. In FIG. 2, two emitter-coupled differential transistor pairs Q1A-Q2A and Q1B-Q2B serve as output amplifiers for respective heads RHA RHB. Transistors Q1A and Q1B and transistors Q2A and Q2B are parallel-coupled such that the emitters of transistors Q1A and Q1B are coupled to current source I1 and to a first terminal of impedance ZE, and the emitters of transistors Q2A and Q2B are coupled to current source I2 and a second terminal of impedance ZE. Thus, impedance ZE, an emitter-coupling impedance, couples the emitters of transistors Q1A and Q2A and the emitters of transistors Q1B and Q2B. The collectors of parallel-coupled transistors Q1A and Q1B and parallel-coupled transistors Q2A and Q2B are connected to voltage supply VS1 (not shown) via respective resistors Rc1 and Rc2. The bases of transistor pair Q1A-Q2A are connected to the first and second terminals of MR head RHA, and the bases of transistor pair Q1B-Q2B are connected similarly to MR head RHA. Series resistor pairs R1A-R2A and R1B-R2B are coupled in parallel to respective heads RHA and RHB. The two junctions formed between the resistors of resistor pairs R1A-R2A and R1B-R2B are coupled to a first input terminal of differential feedback amplifier A, thereby resistively coupling heads RHA and RHB. A second input terminal of amplifier A is coupled to the reference voltage VREF, and the output terminal of amplifier A is coupled to a control terminal of bias current source IBIAS (FIG. 1). The output of current source IBIAS is coupled selectively to the first terminals of respective heads RHA and RHB by a switch (not shown in FIG. 2), and the input of reference current sink IREF (not shown) is coupled selectively to the second terminal of head RHA or RHB by another switch (not shown).
The problem in using the Jove preamp with a plurality of multiplexed heads is that it poorly isolates a selected head from the remaining unselected heads, reducing the capacity of the preamp to reject common-mode signals and noise. To understand the problem, assume that reference voltage VREF equals ground potential, that the inputs of amplifier A are equal, that impedance ZE effectively shorts at the operating frequencies of interest, and that the first and second terminals of the selected head RHA have potentials of xc2x10.1 volts, respectively. Thus, the center voltage of head RHA is zero volts, and the base potentials of transistors Q1A and Q1B are also xc2x10.1 volts, respectively. Further assuming emitter-base-junction (EBJ) drops of 0.7 volts, transistors Q1A and Q2A have respective emitter potentials of xe2x88x920.6 and xe2x88x920.8 volts. Head RHB, on the other hand, is decoupled from source IBIAS and sink IREF, so it conducts no current. Because series resistor pair R1B-R2B is still coupled to the first input terminal of amplifier A, resistor pair R1B-R2B and head RHB are at zero volts. Consequently, transistor Q2B has a base potential of zero volts, which is 0.8 volts higher than its emitter potential of xe2x88x920.8 volts. Hence, the EBJ of transistor Q2B is forward-biased, causing transistor Q2B to conduct current away from the collector of transistor Q2A. The current transistor Q2B conducts away from transistor Q2A prevents the collector currents of transistors Q1A and Q2A from matching. Likewise, when head RHB is selected, the collector currents of transistors Q1B and Q2B will not match. Moreover, when more than two heads are multiplexed like heads RHA and RHB, the current diverted from one side of a differential pair corresponding to a selected head increases proportionately. Hence, the severity of the current mismatch between the transistors of the differential pair will also increase.
The inability or failure to match these currents and the diversion of current degrades performance of the preamplifier in three ways. First, a mismatch between these currents introduces an offset voltage in the output of differential pair Q1A-Q2A or Q1B-Q2B The offset forces the differential pairs to misrepresent the AC voltage across heads RHA and RHB. For example, if the terminal voltages of head RHA or RHB are equal, the output of the respective differential pair Q1A-Q2A or Q1B-Q2B would not be zero volts as it should be, but would be a finite voltage representing the current mismatch. Furthermore, minute variations among the differential transistors would change the offset from pair to pair, complicating any attempts to remove the offset. Second, the mismatch thwarts the ability of the differential pair to reject common-mode noise, i.e. noise appearing equally at the bases of the differential pair. Instead of rejecting common-mode noise as it would absent the mismatch, the Jove differential pair amplifies the noise, increasing the likelihood of misreading data from a magnetic medium. Moreover, since common-mode noise rejection would effectively reject any noise appearing equally at not only the bases of the differential pair, but also the emitters and collectors, the mismatch promotes other noise as well. For example, noise from the power supply inevitably modulates the current sources of the preamnplifier. Thus, noise in current sources I1 and I2 corrupts signals output by the preamplifier. In principle, good common-mode rejection would mute this noise. The current mismatch, however, prevents common-mode rejection, enabling noise from all sources to corrupt output signals and increase data-reading errors. Third, the mismatch, resulting from current being diverted from the differential pair, reduces the gain of the differential pair, i.e. the preamplifier gain. In particular, the gain of the preamplifier depends directly on the sum of the collector currents of the differential pair. Thus, because any diversion of current away from the pair reduces the sum of these currents, any diversion reduces preamplifier gain. Moreover, adding more heads further reduces preamplifier gun and common-mode rejection.
Another problem with the Jove preamp stems from its noisy bias current source and feedback loop. To better understand the problem, FIG. 3 illustrates the specific circuitry of the Jove feedback loop. As shown in FIG. 3, the feedback loop comprises a differential pair consisting of transistors Q6 and Q7, resistors R3-R5 diodes D1 and D2, current source I4, capacitor C1, and controlled current source IBIAS formed by Darlington-paire A transistors Q8 and Q9. Again, switching circuitry is not shown for sake of clarity. The bases of transistors Q6 and Q7 serve as respective first and second inputs of amplifier A while the collector of transistor Q6 serves as the output terminal of amplifier A. In the feedback loop, a significant source of noise is the base shot noise of transistor Q9. This shot noise shunts in part through the emitter of transistor Q8. Transistor Q8 has an emitter impedance that varies inversely with the transistor gain factor beta and directly with the sum of the small-signal emitter and base resistances re and rb of transistor Q8. Emitter resistance re is negligible in comparison to base resistance rb, and the base resistance, or reactance, rb depends inversely on capacitor C1. Thus, to promote shunting, capacitor C1 should be large. In the Jove preamp, however, capacitor C1, an integrated capacitor, is maximally limited to about 50 picofarads. Therefore, if beta ranges from 50 to 200 and the preamp operates at one megahertz, capacitor C1 (having a reactance of 3175 ohms) dictates an approximate shunting impedance ranging between 15 and 60 ohms. Such a range is too high to effectively shunt the base shot noise of transistor Q9.
A further problem with the Jove preamp concerns its frequency response characteristics. Generally, feedback systems should have a high open-loop gain to minimize the magnitude of a steady-state error between an actual and a desired system output. High open-loop gains, however, diminish the phase margin, or stability, of a feedback system. In other words, although higher gains reduce the magnitude of the error in a feedback system, the reduction usually comes at the expense of stability. One way to buttress stability and still maintain high gain and low error is to adjust pole frequencies in the transfer function of the feedback system. Adjusting pole frequencies to increase stability is known as frequency compensation.
In Jove, the frequency response problem is two fold: one, the open-loop gain of the feedback loop is too low to adequately minimize error in the system and two, the gain that is present is undercompensated, leading to inadequate stability of the feedback loop. With respect to FIG. 3, performance of the circuit as a feedback amplifier is inadequate because the collectors of transistors Q6 and Q7 are not biased symmetrically, i.e. at the same voltage and current levels. Symmetrical biasing facilitates error minimization by enabling differential pair Q6-Q7 to respond to an actual difference between desired center voltage VREF and the actual center voltage input at the base of transistor Q7. Asymmetrical, or mismatched, biasing causes differential pair Q6-Q7 to misperceive, or undervalue, the feedback error of the loop. The net effect of misperceiving the feedback error is to lessen the response of the feedback loop to the actual error, i.e. to reduce the gain of the loop. Second, the gain of the loop, controlled by resistors R3 and R4, remains insufficiently compensated to offset instability concerns. One reason for the higher than desirable gain (at least from a stability perspective) is the inadequate degeneration resistance provided by resistors R3 and R4. The bias voltage of resistors R3 and R4, fixed by current source I4, frustrates the degenerative effect necessary to ensure a more stable feedback loop.
In sum, the Jove preamp is beset by numerous deficiencies. The present invention recognizes and addresses each of these and others not expressly detailed to provide an improved preamplifier having a programmable bias reference current, better isolation among multiple heads, a higher common-mode rejection ratio, and a more stable and efficient bias control system.
A preamplifier for an MR element having first and second ends comprises first and second feedback loops which control the biasing of the MR element. The first feedback loop responds to a center voltage of the MR element to control a current source coupled to the first end of the MR element, and the second feedback loop responds to an output voltage across the MR element to control a current sink coupled to the second end of the MR element.
In one embodiment, the preamplifier further comprises a multiplexer coupled between first and second MR elements and the first and second feedback loops. The multiplexer responds to a first input mode to couple the first center voltage to the first feedback loop and decouple the second center voltage from the first feedback loop and responds to a second input mode to couple the second center voltage to the first feedback loop and decouple the first center voltage from the first feedback loop.
In preferred embodiments, the first feedback loop includes a differential feedback amplifier, and the second feedback loop includes a differential-to-single-ended voltage converter. Preferably, the voltage converter comprises first and second opposing transconductance amplifiers.
According to one aspect of the present invention, the preamplifier biases the MR element by deriving a center voltage of the MR element and a signal based on a difference between the center voltage and a reference voltage. Using the difference signal, the preamplifier controls a current source coupled to a first end of the MR element. The output voltage of the MR element operates a controllable current sink coupled to the second end of the MR element.
According to another aspect of the present invention, the MR element provides a differential output voltage, which is converted to a single-ended voltage. A parallel circuit has first and second symmetrical branches coupled between first and second voltage sources. The differential output voltage drives a first transconductance amplifier to produce a voltage imbalance between the first and second symmetrical branches. A second transconductance amplifier operates according to the voltage imbalance to restore balance of the first and second symmetrical branches. A differential amplifier operates on the voltage imbalance to produce a single-ended voltage.